The present invention relates to a charged particle beam apparatus that irradiates a specimen with a charged particle beam and detects secondary charged particles generated from the specimen, and relates to a control method for that apparatus.
As an apparatus for observing a circuit pattern formed on a specimen such as a semiconductor wafer, there is a charged particle beam apparatus. A charged particle beam apparatus is an apparatus that irradiates a specimen with a primary charged particle beam, detects secondary charged particles generated by the irradiation, and expresses and displays these secondary charged particles as an image. In the case where the primary charged particle beam is an electron beam, the charged particle beam apparatus is called a Scanning Electron Microscope (hereinafter, abbreviated as SEM).
In the case of an SEM, when an electron beam goes deeply into a specimen, resolution of secondary charged particles becomes lower. Further, quite a few of specimens have low tolerance to an electron beam. Thus, some SEMs are provided with a mechanism for applying retarding potential to a specimen.
Among specimens, there are specimens that are electrified by themselves. For example, in the case where a specimen is a semiconductor wafer, plasma etching processing or resist coating processing are considered to be a cause of electrification of the specimen. However, it is impossible to explain all the causes of electrification. In any way, a stationary charge that remains even when a specimen is grounded is considered to be a cause of such electrification. Such electrification deflects the path of an irradiated electron beam or shifts a focused focal point. As a result, it takes a time for adjusting an electromagnetic lens or the like to adjust the focus position once again, and throughput of measurement of a minute pattern is largely reduced.
Thus, the below-mentioned Patent Document 1 discloses a technique of estimating an electric potential of a semiconductor wafer. This technique detects potentials at a plurality of points on a line passing through the center of a semiconductor wafer in the course of carrying the semiconductor wafer to a specimen exchange chamber by a delivery robot, and obtains an electric potential distribution function of the semiconductor wafer. In detail, first, potentials at a plurality of points in the radius of a semiconductor wafer are detected, and the potentials at these points are approximated by a quartic function, and then a potential distribution function is obtained by rotating this quartic function about the wafer's center that is taken as the origin. Then, this potential distribution function is used to estimate a potential at a observation point, and the estimated value is fed back to the retarding potential. As a result, focusing is performed in a short time.
Patent Document 1: International Publication WO2003/007330
The technique described in Patent Document 1 however assumes that potential distribution on a wafer becomes concentric, or in other words, 1-fold rotationally symmetric. Thus, in the case where the actual potential distribution is not rotationally symmetric, a large difference occurs between the actual potential distribution and the estimated potential distribution and the retarding potential can not be set to a suitable value, so that measurement at the observation points takes a time.
The present invention focuses on this problem of the conventional technique. And, an object of the present invention is to provide a charged particle beam apparatus that precisely estimates the wafer's potential distribution due to static electrification, and can set setting parameters of a charged particle beam optical system such as a retarding potential and the like to suitable values, and to provide a control method for that apparatus.